The present invention relates to a recording apparatus for recording a two-dimensional image containing a half tone on a photosensitive sheet in accordance with an image signal. The invention also pertains to a recording method using such an apparatus.
As a prior art example of such a recording apparatus, an apparatus which photographs an image displayed on the screen of a CRT is disclosed in U.S. Pat. No. 4,438,453, for instance. The recording apparatus using the CRT cannot easily be reduced in size, and non-uniformity of a fluorescent material of the CRT incurs non-uniform illumination on the screen, making accurate half-tone recording difficult. Further, the image displayed on the screen is distorted in the marginal portions thereof, and the bright spot becomes larger with an increase in brightness, that is, resolution varies with the brightness.
As a half-tone recording apparatus free from such defects, there is disclosed in Japanese Patent Application Laid Open No. 103122/86 a printer which displays a two-dimensional image by liquid crystal cells arranged in a matrix form and records the image by forming it on a photosensitive sheet through exposure. In this Japanese application it is described that, in the case of recording n gradations, exposure is carried out (n-1) times each for the same period of time and each liquid crystal cell is driven to transmit therethrough light (ON) by the same number of times as the value of gradation of the corresponding pixel. Moreover, it is also disclosed that, to reduce the number of times of exposure involved for each image recording, the light transmittance of each liquid crystal cell during exposure is varied in proportion to powers of 2, i.e. 2.sup.0, 2.sup.1, 2.sup.2, . . . and a combination of exposures weighted by such variations of the transmittance are used for recording a plurality of gradations. According to the recording apparatus described in this Japanese application, however, it is difficult to implement accurate gradation recording, because the scatter of response of the liquid crystal cells leads to dispersion of the amount of exposure and because variations in the intensity of light from a light source also cause variations in the amount of exposure. Furthermore, the optical response characteristics of the matrix-arranged liquid crystal cells which are driven under the same condition are scattered and readily affected by temperature. Accordingly, there is a variation in the amount of light to which a pixel corresponding to each liquid crystal cell is exposed. Even if a liquid crystal cell array which is free from the scatter of the optical response characteristics of individual liquid crystal cells could be obtained, voltage applied to the cells varies due to a voltage drop by the resistance of an access line of the matrix, resulting in dispersion of the response speed of the cells. Therefore, it is difficult to appey an accurate amount of light corresponding to the gradation of the image to a pixel area on the photosensitive sheet corresponding to each liquid crystal cell.